In the manufacture of semiconductor integrated circuits, a photoresist film is formed over a semiconductor wafer. The photoresist film may be irradiated so that some regions of the photoresist film are either harder or easier to dissolve in aqueous base developer. As a result, a pattern can be repeatedly transferred to the semiconductor wafer via the photoresist film. After developing, the photoresist film may be used as a mask for etching desired features in the underlying layers of the semiconductor wafer.
Advances in photolithography techniques utilized to transfer patterns to photoresist have enabled increasingly smaller patterns to be transferred. As a result, smaller integrated circuit features can be formed in integrated circuits. Thus, more elements can be put in a given area on a semiconductor integrated circuit. One result of these advances has been to reduce the cost of integrated circuits.
One advanced photolithography technology is extreme ultraviolet technology (EUV). EUV uses short wavelength radiation, typically in the spectral region between about 1 nm to about 40 nm, in order to carry out projection imaging. Currently, most EUV work is carried out in a wavelength region of about 13.5 nm. However, as a result of the plasmas used to produce EUV, there is a large component of the radiation that is “out-of-band,” that is, there is a large component of the radiation that has a wavelength different from the target wavelength of the EUV source (typically about 13.5 nm, +/− about 0.05 nm). The optics in an EUV system tend to exhibit chromatic aberrations, which are wavelength dependent. As a result, it is desirable to tightly control the wavelength range of the source in an EUV system.
FIG. 1 provides a graph of a sample spectral output of a typical EUV source, and plots intensity of light in a.u. (“arbitrary units”) versus radiation wavelength in nanometers. As seen in FIG. 1, there is a non-negligible fraction of EUV radiation that can be out-of-band. EUV photoresists are not only sensitive to EUV radiation, but also to out-of-band radiation, and, especially to deep ultraviolet (DUV) radiation with a wavelength between about 200 nm and about 300 nm, and usually at about 248 nm. Thus, although all out-of-band radiation may be problematic, DUV out-of-band radiation is particularly problematic since EUV photoresists are sensitive to DUV light. Exposure of the EUV photoresist to out-of-band radiation typically results in unwanted background exposure of the resist called “flare.” Flare among other things hurts the resolution of the resist, reducing contrast with respect to unexposed areas, and compromising the ability to etch patterns of sufficiently small sizes.
As is well known, EUV imaging systems are entirely reflective systems that function based on mirrors or reflecting surfaces coated with multilayer thin films (ML's), typically Mo and Si, sometimes capped with Ru, although other materials may be used for multilayer thin films and for the capping layer. The ML's in an EUV imaging system do not mitigate the problems caused by out-of-band radiation. ML's used in EUV lithography reflect out-of-band radiation in the DUV range almost as well as radiation in the EUV range. Thus, out-of-band radiation is in no way attenuated by the presence of the ML's.
To prevent DUV radiation from reaching the wafer being etched using EUV lithography, the prior art proposes the use of an optical element typically referred to as a “Spectral Purity Filter” (SPF). An SPF transmits EUV light but blocks out-of-band radiation, specifically DUV light. However, to the extent that all materials tend to absorb EUV, the presence of the SPF is expected to result in a reduction of the EUV radiation reaching the wafer by about 50%, disadvantageously reducing a throughput of the system and possibly requiring higher power sources, either of which would drive up the cost of ownership with respect to EUV lithography.
Prior art methods of alleviating problems associated with flare in EUV systems disadvantageously reach their goal only by compromising the throughput of the system as a whole.